I often snorkel with my non diving wife so we can do this sport together on our ventures. I have taught her to use a weight belt so we can get down routinely to 20 to 40ft. You touched on an important point I taught her, make sure you have enough left in your lungs to get back up. Being weighted and neutrally buoyant you have to kick a lot harder and seems longer than going down. I learned quickly when that natural reflex to breath gets overwhelming and you still have even 10ft to go a bit of panic happens and adrenalin kicks in and you shift gears to get back to surface. It’s a timing thing you learn with experience, when you push it to limits when it’s time to go you turn up and kick it. I started her shallow going to 10ft and in no time she can go like a pro with me and I don’t have to focus on her, just what we see.